One record expected to be broken at the London Summer Olympics is the size of its audience — an expected 4 billion people. For advertisers, that's a golden opportunity. But there are also strict rules about who can use the Olympics to promote their products.

One of those rules is known as the "blackout," a period starting Wednesday in which athletes competing in the games may not appear in any advertising by companies that are not official Olympic sponsors.

To understand what this means, consider Michael Phelps: Subway has long sponsored the Olympic swimmer, but it's not an Olympic sponsor. That means no Subway ads featuring Phelps can air between July 18 and Aug. 15. But this Head & Shoulders commercial of Phelps washing his hair is fine — Head & Shoulders is owned by Procter & Gamble, which is an Olympic sponsor.

Corporations have paid a lot of money to officially partner with the London Games. According to Lisa Baird, chief marketing officer for the U.S. Olympic Committee, that's why this and other rules exist.

"It's a great protection for the sponsors, and it is a commitment that the athletes make to the International Olympic Committee," Baird says.

Athletes who violate the rules risk being sanctioned.

Fighting Back Through Ambush Marketing

Peter Carlisle, who heads the sports management company Octagon, has secured a number of endorsement deals for Olympic athletes, including Michael Phelps. He says he's frustrated by the blackout rule. As the USOC limits an athlete's ability to raise money, Carlisle says, it's also using that athlete's publicity rights to raise money for itself.

"It's a bit contradictory," he says.

Of course, some athletes and advertisers just find clever ways to get around the rules. It's called "ambush marketing," or promoting an affiliation with an event or property without permission. One legendary example is from the 1996 Olympics in Atlanta when sprinter Linford Christie showed up at a news conference wearing contact lenses with the Puma logo on them. Every image of Christie from that conference came with a built-in Puma ad, but Puma hadn't paid for sponsorship that year — its competitor Reebok had.

Knitting And The Challenge Of Enforcement

This year, according to the USOC's Lisa Baird, the enforcers are on high alert. You can't use anything related to the Olympics if you haven't paid for it.

"You may not use any of our marks," she says. "You may not use our licensed footage. You may not use insinuation even to really convey that you might have a relationship with either the committee or the U.S. Olympic Team."

And those rules apply to everyone — even knitters. A social networking site for knitters called Ravelry recently got a cease and desist letter from the USOC over an annual event they call the "Ravelympics." The letter said the name was an infringement, so the knitters changed the name to the "Ravellenic Games." But that slap on the wrist got a lot of attention, and the USOC has since apologized.

"We have a great respect for American handcraft, and it was not our intention to be insensitive at all," Baird says.

According to Baird, they are still going to be "very tough" on anyone who uses the Olympics without permission. But policing all of the different platforms, and all of the athletes on all of those platforms, will be quite a challenge.

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